Hitherto, as a measure against external vibration that is to be applied to an object subjected to seismic isolation such as a precision instrument, an electronic device, or an artwork at the time of transportation of the object subjected to seismic isolation or under influence of an earthquake on the object subjected to seismic isolation which is placed in a building, a seismic isolation device has been used for the purpose of isolating those objects subjected to seismic isolation from vibration of a floor surface. Further, as the seismic isolation device, there have been known a horizontal seismic isolation device configured to absorb vibration in a horizontal direction, a vertical seismic isolation device configured to absorb vibration in a vertical direction, and a three-dimensional seismic isolation device having functions of both the horizontal seismic isolation device and the vertical seismic isolation device.
As a structure of the vertical seismic isolation device among the above-mentioned seismic isolation devices, as disclosed in Patent Literature 1, there has been known a structure in which seismic isolation units each formed of a four-joint link that is so-called pantograph mechanism are arranged between a building base being a movable frame and a base being a fixed frame. The seismic isolation units illustrated in FIG. 1 of Patent Literature 1 each include four links arranged in a rhombic shape. A tensile coil spring is arranged between a pair of link support points. Between a fixed end coupled to the fixed frame and a free end coupled to the movable frame, an urging force is applied in a direction of separating the fixed end and the free end away from each other. With this configuration, when vibration in the vertical direction is applied between the movable frame and the fixed frame, the free end provided on a side opposed to the fixed end moves horizontally so that the movable frame can freely move in the vertical direction under a state of being isolated from the vibration of the fixed frame.